Well dogma13, pretty much spelled it out the process, and I think I can provide a how to guide. Your dog sounds very much like mine??
My guy greeted company with a Cold Hard stare and a low growl??? Clearly not a people person???
Ok then no big deal, meet and greet was off the table "Place" in the home and keeping people out of his face it was. Train place first, use a muzzle and drag leash, in public if need be, you'll feel better and the dog is less likely to add a fool. None of the Cesar meet and greet at the door to negotiate the entrance of people into the home, for me. Dog goes to "Place" and stays there ,your job is to keep company out of his face! If you can't or won't do that ... in the Crate you go!
So step one work with the dog in front of you ... if he seems not to be a fan of people ...no big deal, if you are willing to accept that?? Dealing with it looks like the second link here:
The rest is as "Dogma" explained you" need to learn to say "No, you can't pet my dog" show him how you expect him to behave! When meeting people his job is to do "nothing!" "Walking the dog to find people to ignore" is required, use a muzzle if you chose, a few encounters over a period of time, dog at your side or behind you and the dog will get it. Dad/Mom has my back ... no problem here. Once you understand and recognize how your dog looks when he is calm, then you can lose the muzzle ... the term we came up with here is "Bubble Dog" a dog that needs a protective "Bubble" around him until he is safe in public ...no big deal.
Part two ... The Place Command and Sit on the Dog" they train calmness into a Dog. And while others may disagree ... I used "Place" in the home with Company. My Dog goes to "Place" and does not move, period end of story! My job ... keep company out of his face! Don't even freaking ask! Family friends whoever ... I don't care. And I was eyes on, if you can't ensure that ... put him in his Crate. Not a fan of tricking people into my Dog's face with treats myself, "my" way worked out fine .. so I'm good with that.
"Sit on the Dog" ... find an area with distractions (People) off to the side and you people watch. Take the leash and sit on it, tell the dog "nothing" no corrections, no commands. You stop and you sit ... this is how it is dog, deal with it as it were. Details are here:
Fearful, Anxious or Flat Crazy "The Place CommanD - Boxer Forum : Boxer Breed Dog Forums
So if your willing to work with the dog in front of you and not treat him like a lab and stuff ie "my dog likes everybody!!" Then ... the people thing is no big deal ...been there done that ... worked out fine. As always ... ask questions and welcome aboard.